In the bent-axis hydraulic pump motor in which a stroke distance of a piston rod is changed by changing an inclination angle (inclined rotation angle) of an axial center of a cylinder block with respect to an axial center of a drive shaft, there has been a technique of connecting the cylinder block to a retainer plate of the drive shaft by using a constant velocity joint as a mechanism to synchronously rotate the cylinder block and the drive shaft. The constant velocity joint is, for example, a component in which a plurality of balls held in a cage is disposed between a sphere-shaped inner joint element provided at one end portion of a cylinder block and a cylindrical recessed portion provided at a retainer plate serving as an outer joint element, and the cylinder block and the retainer plate are connected by interposing the balls between ball grooves of the inner joint element and ball grooves of the outer joint element.
To form a constant velocity joint between a cylinder block and a retainer plate, all of balls are needed to be disposed at positions on a bisecting plane relative to an inclined rotation angle between the cylinder block and the drive shaft. Therefore, in the related art, setting is made such that a cage is kept on a bisecting plane of an inclined rotation angle between a cylinder block and a drive shaft by: offsetting a center of a sliding contact surface between the cage and an inner joint element and a center of a sliding contact surface between the cage and a retainer plate in opposite directions along an axial direction of the drive shaft with respect to an intersection (inclined rotation center) between the axial center of the cylinder block and the axial center of the drive shaft; and balancing loads applied to the cage (refer to Patent Literature 1).
However, according to the technology in which the centers of the sliding contact surfaces between the cage and the cylinder block and between the cage and the retainer plate are offset with respect to the inclined rotation center between the cylinder block and the drive shaft, an unnecessary rotational moment is applied to the cylinder block in a case where the cylinder block is rotated. As a result, the cylinder block may be displaced with respect to a valve plate.
Furthermore, according to the structure of Patent Literature 1, heat is generated from the sliding surface because the cage is pressed against the retainer plate by couple generated at the cage during torque transmission, thereby preventing speed increase.
On the other hand, as a constant velocity joint, there has been provided a technology in which a ball groove of an inner joint element and a ball groove of an outer joint element are formed such that a plane including an extending axial line of each ball groove and an inclined rotation center is inclined with respect to an axial center. According to this constant velocity joint, loads applied to a cage from balls can be balanced in the axial direction, and therefore, the cage can be kept at a position on a bisecting plane of an inclined rotation angle between two members without offsetting centers of respective sliding contact surfaces with respect to an inclined rotation center between the two members (refer to Patent Literature 2, for example).